Bubblejet or thermaljet printing technology is a liquid ink variety of the drop-on-demand ("DOD"), as opposed to continuous, inkjet printing technology. Generally, thermaljet technology involves rapidly heating a small volume of ink in an enclosed reservoir. As the ink in the reservoir volatilizes a discrete quantity is ejected through a port or nozzle, which in turn directs the ink droplet onto the desired substrate.
Because so many of the physicochemical characteristics of the ink composition are at play in thermaljet systems, development of such inks requires that careful attention be given to identifying the acceptable ranges for such characteristics. For example, print quality is directly affected by the surface tension, opacity and viscosity of the jet ink composition, as well as the presence of certain additives. Boiling point (volatility) of the fluid carrier(s) is also an important factor.
Early development of ink compositions for thermaljet systems focused on adapting preexisting water-based, or aqueous, compositions to fit the mechanical and physicochemical limitations of thermaljet. While these efforts have met with some success, several drawbacks of the aqueous inks persist. Among the most significant drawbacks of the aqueous inks is their prolonged drying time (10-50 seconds). Because the fluid carrier of these compositions, water, has a slow evaporation rate, the ink remains wet as the paper reaches the stacking tray. As a result, the ink is often smudged as other pages are stacked on the freshly printed page or when the page is otherwise handled.
While mechanical means for curing this problem have been devised, they have drawbacks of their own. For example, one approach feeds the printed page through a stream of warm air on its way to the stacker; this adds both expense and bulk to a printer that counts among its most significant advantages compact size and portability. Yet another approach is simply to lengthen the paper path, giving the ink more time to dry before the pages are stacked, this also adds undesired bulk to the system.
Another approach to creating a smudge-proof image is the addition of chemical penetrants. These penetrants, e.g., butyl cellosolve, increase the penetration rate of the ink thereby shortening the drying time. However, penetrants may reduce print quality, e.g., feathering, due to increased capillary action and hence lateral spreading of the ink. Furthermore, because today's office systems utilize such a variety of paper types and quality, each varying in fiber construction, the amount of feathering will vary with the type of paper utilized, thereby reducing uniformity of print quality across paper types.
Another drawback of the aqueous ink compositions is the difficulty involved in creating a waterfast image. Because the dye colorants of the aqueous compositions are necessarily water soluble they are subject to smearing when contacted with water even after the ink has dried.
Yet another drawback associated with the use of aqueous thermaljet ink compositions is the need for additives in the ink. Because the ink remains exposed to the atmosphere between uses, evaporation of the fluid carrier can result in deposition of solids inside the nozzle or reservoir, which in turn can clog the nozzle or in less extreme cases simply cause a reduction of print clarity. To reduce clogging many ink compositions contain humectants, which are additives that improve moisture retention. While improved moisture retention rectifies one problem, it exacerbates the problem observed with the long drying time of aqueous inks.
The dye colorants themselves used in the aqueous inks also present problems. These dyes, e.g., azo dyes, are generally susceptible to thermal decomposition. Because many of the dyes, per se, are not water soluble, they often contain polarizing substituent groups, e.g. sulphate moieties, to improve water solubility. When thermal decomposition occurs the molecular dye is cleaved into two or more molecular fragments. Often, only one of these fragments will retain the polarizing substituent, and hence water solubility. The other fragments, no longer water soluble, precipitate out of solution to either clog the nozzle or build up decomposition products on the heating element in the reservoir. Such a build up, referred to as kogation, can act as an insulator reducing the heating efficiency of the element, which in turn will impair bubble formation, and may ultimately burn out the heating element. Such loss of individual inkjets reduces the print quality of the system.
Similarly, biocides, another common additive of aqueous ink compositions, are likewise susceptible to thermal decomposition, and accordingly the drawbacks associated therewith.